I have a 100mm x 100mm ND 3 filter. What filter/lens adapters might be available to attach it to my Rokinon 14 mm lens?

I suggest to use filter with a size of 150x150mm. there are several products on the market like Haida, Rollei etc. They all also supply various holders. They question is whether you will find a holder for your lens. There are some dealers with special holders like:
https://shop.logodeckel.de/
https://foto-jaeker.de/

But isnt the Rokinon the same as the Samyang? If so, it might be a easier.